1. Field of the Invention
This invention relates to a device for impregnating strands or ribbons of natural fibers, in particular of linen, with a specific aqueous polymer dispersion in order to consolidate the fibers at the core of the fiber bundle and to improve their mechanical strength without any need for twisting. The invention also relates to the treated fibers and to their use in composite materials.
Natural fibers of linen, hemp or sisal and, in particular of linen, are not continuous fibers but discontinuous fibers connected to one another by transverse fibrils ensuring their strength. However, this strength is fairly low, complicating the implementation and the use thereof in finished products. Therefore, it is advantageous and even necessary to reinforce the mechanical strength of the strands or yarns or ribbons of natural fibers, in contrast to the case of glass or carbon fibers which are continuous.
2. Description of the Related Art
Concerning the use of such natural fibers in textiles, it is known to twist them so as to form a yarn having a sufficient mechanical strength.
The challenge is to obtain this improved mechanical strength of the strand of fibers but without any twisting of the fibers. When one wants to make a natural reinforcement in order to fabricate composites, one tries to preserve to a maximum extent the intrinsic qualities of the natural fibers, notably of the linen fibers. Thus, it is recommended not to twist the fibers. In this case, the strands of fibers, that is to say bundles of fibers or ribbons of natural fibers, then consist of fibers for technical use having better mechanical properties than the twisted fibers for textile use, since they have not been damaged by the twisting step.
Consequently, the obtention of a strand or of a ribbon of technical natural fibers that are sufficiently resistant so as not to break during their fabrication requires the production of strands or ribbons of fibers with high yarn count (expressed in tex) that are clearly higher than what is customary with glass or carbon fibers. As a reminder, one tex corresponds to a yarn count or linear weight (per unit of length) equal to 1 g/km or 10-6 kg/m. This required higher yarn count results in unidirectional fibrous reinforcements or nonwovens, or in coarser fabrics, comprising imperfections and thus affecting the final mechanical strength of the composites containing natural fibers such as linen fibers. With this type of reinforcement based on natural fibers, it is moreover not possible to produce light sandwich panels comprising skins made of a composite containing natural fibers, in particular containing linen fibers, of very small thickness.
Thus, the purpose of a ribbon or strand of lower yarn count made of linen fibers is to produce composites containing natural fibers that are nearly continuous and have a greater mechanical strength, and to produce light sandwich panels comprising composite reinforcements based on technical natural fibers, which have better performances than those produced from textile fibers.
Thus, there is a need for consolidating and reinforcing said strands of fibers without using any twisting of the fibers and by looking for a binder that is capable of penetrating to the core of the strands of said fibers to bind the fibers to one another with sufficient cohesion to improve the mechanical strength of the strand.
Thermoplastic composites already have the advantage over composites with a thermosetting matrix of being recyclable and easy to work or shape. The reinforcement with natural fibers adds an additional advantage of the recyclability due to the plant origin of the fibers. This is an important element to consider in the ecological context where one aims to use environmentally friendly, renewable raw materials.
In addition, in comparison to the thermoplastic composites with glass fiber reinforcement, the use of thermoplastic composites with fibrous reinforcement of natural origin, in particular based on linen fibers, makes it possible to reduce the weight of said composites while achieving an equivalent performance. Indeed, the density of the linen fibers (1.5) is approximately forty percent lower than that of the glass fibers. Hence the increasing interest in and need for the ability to fabricate strands or ribbons of natural fibers, in particular of linen, with consolidation between the fibers of the bundle and improved mechanical strength without twisting.
The patent GB 512 558 describes the treatment of cotton fibers by a dispersion of rubber or synthetic resin having a low polymer content ranging from 2 to 10%, without any need to twist the cotton yarns for tensile mechanical strength. After impregnation under pressure, the excess of dispersion is eliminated by a pressurized air jet, with few polymer particles remaining on the fibers after elimination of the excess and drying of the fibers. No data on polymer content remaining between the fibers is specified, and no precise example is cited on the conditions of obtention and on the precise results obtained. This document also does not describe the technical problem of natural fibers such as linen fibers, as stated above, for the purpose of reinforcing thermoplastic matrixes of thermoplastic composite materials.
On the other hand, the patent EP 324 680 describes a device for preparing a reinforced thermoplastic semi-finished product, as a material based on polypropylene reinforced with glass fibers. According to this document, the wetting of the reinforcement fibers is better the more the fibers remain in the unit state and not in the state of strands or base yarns. More particularly, the method described in connection with this device includes the preparation of an aqueous coating composition comprising a resin in the fragmented state and a viscosity regulating agent, and optionally additives, followed by coating of a surface of a mat of reinforcement fibers dispersed in the unit state with the aqueous coating composition, and then drying in order to obtain a semi-finished product in sheet form, optionally followed by melting of said resin. No technical problem connected with the natural fibers such as linen fibers is mentioned in this document, which relates more to that of a semi-finished product based on a polyolefin reinforced with glass fibers.
As to the patent FR 2 223 173, it describes a device for preparing sheets or ribbons of fibers impregnated with resin from an aqueous dispersion of resin, in particular a thermosetting resin, with dispersion after thickening by means of a thickening agent. Again, the fibers concerned are not natural fibers such as linen fibers, and neither the problem presented nor a solution thereof is suggested.
This invention makes it possible to solve the technical problem stated above with respect to the prior art with a specific device for impregnating strands or ribbons of natural fibers wherein this problem arises, this impregnation occurring at the core with a specific aqueous polymer dispersion, thus allowing said polymer to bind at the core of the bundle of fibers, that is to say to one another, the fibers of said strands or of said ribbons in order to consolidate them by the specific impregnation with the fine polymer particles used after melting. This impregnation can lead later and directly to a pre-impregnated fibrous reinforcement, strand or ribbon having a low yarn count that can be used for the fabrication of composite materials.